Aphids or greenfly are insects with an ordinary anatomy, but have an extraordinary life cycle.  The males and females die in the autumn once their eggs have been laid on various trees and shrubs. 

The eggs will survive in hibernation until they hatch in the spring and then tiny young aphids will feed by sucking the sap of new shoots with their piercing needle like mouthparts.  These young, called nymphs, are a miniature version of the adult but without developed reproductive organs.  There is no distinctly different larval stage in aphids.  Unusually, all of these young nymphs are female, no males or eggs will be seen again until autumn. Out of the batch of eggs some will be eaten by birds such a Long-tailed tits or Blue tits but only one egg is needed to start a colony.  It takes just a week before each nymph matures to an adult and then a further week for each adult to produce eighty offspring, all without mating, a process known as parthenogenesis or virgin birth.  Furthermore, each adult gives birth to live young, they are viviparous, no eggs are laid.

This ability to produce as many offspring as quickly as possible, continues throughout spring and summer, until the population becomes overcrowded or the plant becomes less nutritious.  Then another remarkable event happens.  Up to this point all the adults have been wingless, only now are winged adults produced which fly off to find a new food supply.  Why waste resources on producing wings when not yet needed, and those resources can be used instead to produce more aphids!  Sometimes aphids migrate to a new plant of the same species, e.g. the Oak aphid or to a more nutritious summer herbaceous host.  An example of the latter is the Black bean aphid which spends the spring on Spindle or Guelder rose and the summer on broad or runner beans.  To find new food aphids have the ability to migrate the length or breadth of Britain in a matter of days.  They are small, weak fliers but use this to their advantage.  At the hottest time of the day, they walk to the top of the plant and launch themselves in to the rising currents of hot air.  People in hot air balloons have noted that the main insects they see are aphids.  Where the aphids go now is random and depends on the movement of the wind.  At this time they are an important part of the diet of swallows, martins and swifts.  As the air cools the aphids coast downwards, landing at random but using their smell, sight and limited flying capabilities, to home in on their chosen host plant.  If unsuccessful, they repeat the process the next day.  Long, hot spells of summer weather help them to migrate and spread.  If on a herbaceous plant host then in the autumn when it is dying, their last migration will be back to their winter woody host.  Those that arrive now produce males and females which mate and lay eggs and the life cycle is repeated.

Throughout their life aphids damage crops by lowering yield and some species distort and damage young shoots as they feed.  They can also spread plant viruses which makes them our worst group of insect pests.  However, with over five hundred species of aphids in Britain, only a dozen or so are serious pests.  Aphids are important food for many animals including predatory insects such as ladybirds.  During the hot summer of 1976 there was a population explosion of aphids and then Seven spot ladybirds, but I have seen hardly any ladybirds this summer.  Anecdotally, I wonder whether aphids are less common.  Friend or foe I hope that these amazing insects are not in decline.


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